Communication receiver with hybrid equalizer

ABSTRACT

Wireless communication receiver with hybrid equalizer and RAKE receiver. The receiver compares performance of the system for RAKE only and RAKE in combination with equalizer estimates. The receiver enables or disables the equalizer accordingly.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present Application for Patent claims priority to ProvisionalApplication No. 60/475,250 entitled “COMMUNICATION RECEIVER WITH HYBRIDEQUALIZER” filed Jun. 2, 2003, and assigned to the assignee hereof.

BACKGROUND

1. Field

The present invention relates generally to equalization incommunications systems, and more specifically, to a universal receiverincorporating a RAKE receiver and a hybrid equalizer.

2. Background

Communications systems are used for transmission of information from onedevice to another. Prior to transmission, information is encoded into aformat suitable for transmission over a communication channel. Thetransmitted signal is distorted as it travels through the communicationchannel; the signal also experiences degradation from noise andinterference picked up during transmission. An example of interferencecommonly encountered in band-limited channels is called inter-symbolinterference (ISI). ISI occurs as a result of the spreading of atransmitted symbol pulse due to the dispersive nature of the channel,which results in an overlap of adjacent symbol pulses. The receivedsignal is decoded and translated into the original pre-encoded form.Both the transmitter and receiver are designed to minimize the effectsof channel imperfections and interference. For the purposes of thisdisclosure, interference or distortion due to channel imperfections, orany combination thereof will be referred to generally as noise.

Various receiver designs may be implemented to compensate for noisecaused by the transmitter and the channel. By way of example, anequalizer is a common choice for dealing with ISI. An equalizer correctsfor distortions and generates an estimate of the transmitted symbol. Inthe wireless environment, equalizers are required to handle time-varyingchannel conditions. Ideally, the response of the equalizer adjusts tochanges in channel characteristics.

Equalizers are generally complex, tending to increase the powerconsumption of a communication device. A need exists, therefore, for anequalizer design that reduces power consumption. Further, there is aneed for controlling an equalizer so as to operate the equalizer duringsuch channel conditions as result in optimum performance of theequalizer. Still further there is a need to implement an equalizer inparallel with a RAKE receiver, wherein the equalizer only operatesduring specified operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a portion of a rake receiver in a communication system;

FIG. 2A is model of a communication system;

FIG. 2B is a model of a transmission portion of a communication system,including modulation and analog receiver processing;

FIG. 3 is receive data processor in a mobile station;

FIG. 4 is a receiver supporting data communications;

FIG. 5 is a state diagram illustrating operation of a receiver employinga RAKE and hybrid equalizer; and

FIG. 6 is a state diagram illustrating operation of a receiverincorporating multiple operational states.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

Communications systems are used for transmission of information from onedevice to another. Before transmission, information is encoded into aformat suitable for transmission over a communication channel. Thecommunication channel may be a transmission line or free space betweenthe transmitter and the receiver. As the signal propagates through thechannel, the transmitted signal is distorted by imperfections in thechannel. Furthermore, the signal experiences degradation from noise andinterference picked up during transmission. An example of interferencecommonly encountered in band-limited channels is called inter-symbolinterference (ISI). ISI occurs as a result of the spreading of atransmitted symbol pulse due to the dispersive nature of the channel,which results in an overlap of adjacent symbol pulses. At the receiver,the signal is processed and translated into the original pre-encodedform. Both the transmitter and receiver are designed to minimize theeffects of channel imperfections and interference. For the purposes ofthis disclosure, interference or distortion due to channelimperfections, or any combination thereof will be referred to generallyas noise.

Various receiver designs may be implemented to compensate for noisecaused by the transmitter and the channel. In one design, a RAKEreceiver is implemented. In another design an equalizer is used. Instill another design a RAKE receiver and an equalizer are bothimplemented.

RAKE Configuration

A communication system may employ a RAKE receiver to process a modulatedsignal transmitted on the forward link or reverse link. The RAKEreceiver typically includes a searcher element and a number of fingerprocessors. The searcher element searches for strong instances of thereceived signal (or multipaths). The finger processors are assigned toprocess the strongest multipaths to generate demodulated symbols forthose multipaths. The RAKE receiver then combines the demodulatedsymbols from all assigned finger processors to generate recoveredsymbols that are estimates of the transmitted data. The RAKE receiverefficiently combines energy received via multiple signal paths.

The RAKE receiver provides an acceptable level of performance for CDMAsystems operated at low signal-to-noise ratio (S/N). For CDMA systemsdesigned to transmit data at high data rates, such as the HDR system,higher S/N is required. To achieve the higher S/N, the components thatmake up the noise term N need to be reduced. The noise term includesthermal noise (No), interference (Io) due to transmissions by othertransmitting sources and transmissions for other users, and inter-symbolinterference (ISI) that can come from multipath and distortion in thetransmission channel. For CDMA systems designed to operate at low S/N,the ISI component is typically negligible compared to other noisecomponents. However, for CDMA systems designed to operate at higher S/N,the other noise components (e.g., interference from other transmissionsources) are typically reduced and ISI becomes a non-negligible noisecomponent that may have a large impact on the overall S/N.

As noted above, the RAKE receiver provides acceptable performance whenthe S/N of the received signal is low. The RAKE receiver can be used tocombine energy from various multipaths, but generally does not removethe effects of ISI (e.g., from multipath and channel distortion). Thus,the RAKE receiver may not be capable of achieving the higher S/Nrequired by systems operating at higher data rates.

FIG. 1 is a block diagram of an embodiment of rake receiver 100. Due tomultipath and other phenomena, a transmitted signal may reach a receiverunit via multiple signal paths. For improved performance, the rakereceiver is designed with the capability to process multiple (andstrongest) instances of the received signal (or multipaths). For aconventional rake receiver design, a number of finger processors 110 areprovided to process a number of multipaths. Each finger processor 110comprises a finger of the rake receiver and can be assigned to process aparticular multipath.

In a spread-spectrum communication system, such as a Code DivisionMultiple Access (CDMA) system, the received In-phase (I_(IN)) andQuadrature (Q_(IN)) samples from a particular pre-processor (not shown)are provided to a number of finger processors 110 a through 110 l.Within each assigned finger processor 110, the received I_(IN) andQ_(IN) samples are provided to a PN despreader 120, which also receivesa complex PN sequence, PNI and PNQ. The complex PN sequence is generatedin accordance with the particular design of the CDMA system beingimplemented and, for the HDR system, is generated by multiplying theshort IPN and QPN sequences with the long PN sequence by multipliers 138a and 138 b. The short IPN and QPN sequences are used to spread the dataat the transmitting base station, and the long PN sequence is assignedto the recipient receiver unit and used to scramble the data. The PNIand PNQ sequences are generated with a time offset corresponding to theparticular multipath being processed by that finger processor.

PN despreader 120 performs a complex multiply of the complex I_(IN) andQ_(IN) samples with the complex PN sequence and provides complexdespread I_(DES) and Q_(DES) samples to decover elements 122 and 132.Decover element 122 decovers the despread samples with one or morechannelization codes (e.g., Walsh codes) that were used to cover thedata and generates complex decovered samples. The decovered samples arethen provided to a symbol accumulator 124 that accumulates the samplesover the length of the channelization codes to generate decoveredsymbols. The decovered symbols are then provided to a pilot demodulator126.

For a High Rate Packet Data (HRPD) system, such as specified by IS-856,a pilot reference is transmitted during a portion of the forward linktransmission. Thus, decover element 132 decovers the despread sampleswith the particular channelization code (e.g., a Walsh code 0 for theHDR system) that was used to cover the pilot reference at the basestation. The decovered pilot samples are then provided to an accumulator134 and accumulated over a particular time interval to generate pilotsymbols. The accumulation time interval can be the duration of the pilotchannelization code, an entire pilot reference period, or some othertime interval. The pilot symbols are then provided to a pilot filter 136and used to generate pilot estimates that are provided to pilotdemodulator 126. The pilot estimates are estimated or predicted pilotsymbols for the time period when data is present.

Pilot demodulator 126 performs coherent demodulation of the decoveredsymbols from symbol accumulator 124 with the pilot estimates from pilotfilter 136 and provides demodulated symbols to a symbol combiner 140.Coherent demodulation can be achieved by performing a dot product and across product of the decovered symbols with the pilot estimates. The dotand cross products effectively perform a phase demodulation of the dataand further scale the resultant output by the relative strength of therecovered pilot. The scaling with the pilots effectively weighs thecontributions from different multipaths in accordance with the qualityof the multipaths for efficient combining. The dot and cross productsthus perform the dual role of phase projection and signal weighting thatare characteristics of a coherent rake receiver.

Symbol combiner 140 receives and coherently combines the demodulatedsymbols from all assigned finger processors 110 to provide recoveredsymbols for a particular received signal being processed by the rakereceiver. The recovered symbols for all received signals may then becombined, as described below, to generate overall recovered symbols thatare then provided to the subsequent processing element.

Searcher element 112 can be designed to include a PN despreader, a PNgenerator, and a signal quality measurement element. The PN generatorgenerates the complex PN sequence at various time offsets, possibly asdirected by a controller (not shown), which are used in the search forthe strongest multipaths. For each time offset to be searched, the PNdespreader receives and despreads the I_(IN) and Q_(IN) samples with thecomplex PN sequence at the particular time offset to provide despreadsamples. The signal quality of the despread samples is then estimated.This can be achieved by computing the energy of each despread sample(i.e., I_(DES) ²+Q_(DES) ²) and accumulating the energy over aparticular time period (e.g., the pilot reference period). Searcherelement performs the search at numerous time offsets, and the multipathshaving the highest signal quality measurements are selected. Theavailable finger processors 110 are then assigned to process thesemultipaths.

The design and operation of a rake receiver for a CDMA system isdescribed in further detail in U.S. Pat. No. 5,764,687, entitled “MOBILEDEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM,” and U.S. Pat. No. 5,490,165, entitled“DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVINGMULTIPLE SIGNALS,” both assigned to the assignee of the presentinvention.

In one embodiment, a number of forward link signals are received by kantennas and processed to generate sample streams x₁ (n) through X_(K)(n). Thus, a number of rake receivers may be provided to process the ksample streams. A combiner may then be used to combine the recoveredsymbols from all received signals being processed. Alternatively, one ormore rake receivers can be time division multiplexed to process the Ksample streams. In such a Time Division Multiplex (TDM) rake receiverarchitecture, the samples from the k streams may be temporarily storedto a buffer and later retrieved and processed by the rake receiver.

For each received signal, rake receiver 100 may be operated to processup to L multipaths, where 1 represents the number of available fingerprocessors 110. Each of the L multipaths corresponds to a particulartime offset identified with the assistance of searcher element 112. Acontroller or searcher element 112 may be designed to maintain a list ofthe magnitude of the strongest multipath (α_(Ji)) and corresponding timeoffset (τ_(i)) for each of the k received signals being processed.

In the combination receiver configuration having a RAKE and anequalizer, these magnitudes and time offsets can be used to initializethe coefficients and scaling factors of equalizer, as described above.In one implementation, the magnitude of each multipath of interest canbe computed as the square root of the accumulated energy value dividedby the number of samples (N) used in the accumulation.

Equalizer Configuration

An equalizer is a common choice for dealing with ISI. An equalizer maybe implemented with a transversal filter, i.e. a delay line withT-second taps (where T is the symbol duration). The contents of the tapsare amplified and summed to generate an estimate of the transmittedsymbol. The tap coefficients are adjusted to reduce interference fromsymbols that are adjacent in time to the desired symbols. Commonly, anadaptive equalization technique is employed whereby the tap coefficientsare continually and automatically adjusted. The adaptive equalizer usesa prescribed algorithm, such as Least Mean Square (LMS) or RecursiveLeast Squares (RLS), to determine the tap coefficients. The symbolestimate is coupled to a decision-making device such as a decoder or asymbol slicer.

The ability of a receiver to detect a signal in the presence of noise isbased on the ratio of the received signal power and the noise power.This ratio is commonly known as the signal-to-noise power ratio (SNR),or the carrier-to-interference ratio (C/I). Industry usage of theseterms, or similar terms, is often interchangeable, however, the meaningis the same. Accordingly, any reference to C/I herein will be understoodby those skilled in the art to encompass the broad concept of measuringthe effects of noise at various points in the communications system.

Typically, the C/I may be determined in the receiver by evaluatingsymbol estimates of a known transmitted symbol sequence. This may beaccomplished in the receiver by computing the C/I for the transmittedpilot signal. Since the pilot signal is known, the receiver may computethe C/I based on the symbol estimates from the equalizer. The resultantC/I computation may be used for a number of purposes. In communicationssystems employing a variable rate data request scheme, the receiver maycommunicate to the transmitter the maximum data rate it may supportbased on the C/I. Furthermore, if the receiver includes a turbo decoder,then depending on the transmitted constellation, the Log LikelihoodRatio (LLR) computation needs an accurate estimate of the C/I.

Equalizers in wireless communication systems are designed to adjust totime varying channel conditions. As the channel characteristics change,the equalizer adjusts its response accordingly. Such changes may includevariations in the propagation medium or the relative motion of thetransmitter and receiver, as well as other conditions. As discussedhereinabove, adaptive filtering algorithms are often used to modify theequalizer tap coefficients. Equalizers that employ adaptive algorithmsare generally referred to as adaptive equalizers. Adaptive algorithmsshare a common property: adaptation speed decreases as the number ofequalizer taps increases. Slow adaptation impacts the tracking behaviorof adaptive equalizers. A “long” equalizer, i.e., an equalizer having alarge number of taps, is desirable as long equalizers more accuratelyinvert channel distortions resulting in good steady state performance.Long equalizers, however, react more slowly to channel variationsleading to poor transient behavior, i.e., poor performance when thechannel is rapidly varying. An optimum number of taps balances suchconsiderations and compromises between good steady-state performance andgood transient performance.

In practice, determining the optimum number of taps is difficult as theoptima depends on a variety of conditions and goals, including but notlimited to, the instantaneous response of the channel, and the rate ofvariation of the channel. So it is difficult to determine, a priori, theoptimum number of taps if the equalizer is to be used on a variety ofchannels, in a variety of time-varying conditions.

FIG. 2A illustrates a portion of the components of a communicationsystem 200. Other blocks and modules may be incorporated into acommunication system in addition to those blocks illustrated. Bitsproduced by a source (not shown) are framed, encoded, and then mapped tosymbols in a signaling constellation. The sequence of binary digitsprovided by the source is referred to as the information sequence. Theinformation sequence is encoded by encoder 202 which outputs a bitsequence. The output of encoder 202 is provided to mapping unit 204,which serves as the interface to the communication channel. The mappingunit 204 the encoder output sequence into symbols γ(n) in a complexvalued signaling constellation. Further transmit processing, includingmodulation blocks, as well as the communication channel and analogreceiver processing, are modeled by section 220.

FIG. 2B illustrates some of the details included within section 220 ofFIG. 2A. As illustrated in FIG. 2B, the complex symbols γ(n) aremodulated onto an analog signal pulse, and the resulting complexbaseband waveform is sinuosoidally modulated onto the in-phase andquadrature-phase branches of a carrier signal. The resulting analogsignal is transmitted by an RF antenna (not shown) over a communicationchannel. A variety of modulation schemes may be implemented in thismanner, such as M-ary Phase Shift Keying (M-PSK), 2^(M)-ary QuadratureAmplitude Modulation (2^(M) QAM), etc.

Each modulation scheme has an associated “signaling constellation” thatmaps one or more bits to a unique complex symbol. For example, in 4-PSKmodulation, two encoded bits are mapped into one of four possiblecomplex values {1,i ,−1,−i}. Hence each complex symbol γ(n) may take onfour possible values. In general for M-PSK, log₂M encoded bits aremapped to one of M possible complex values lying on the complex unitcircle.

Continuing with FIG. 2A, at the receiver, the analog waveform isdown-converted, filtered and sampled, such as at a suitable multiple ofthe Nyquist rate. The resulting samples are processed by the equalizer210, which corrects for signal distortions and other noise andinterference introduced by the channel, as modeled by section 220. Theequalizer 210 outputs estimates of the transmitted symbols {circumflexover (γ)}(n). The symbol estimates are then processed by a decoder 212to determine the original information bits, i.e., the source bits thatare the input to encoder 202.

The combination of a pulse-filter, an I-Q modulator, the channel, and ananalog processor in the receiver's front-end, illustrated in FIG. 2A andFIG. 2B, is modeled by a linear filter 206 having an impulse response{h_(k)} and a z-transform H(z), wherein the interference and noiseintroduced by the channel are modeled as Additive White Gaussian Noise(AWGN), and coupled to multiplier 208.

FIG. 2B details processing section 220 as including a front endprocessing unit 222 coupled to baseband filters 226 and 228 forprocessing the In-phase (I) and Quadrature (Q) components, respectively.Each baseband filter 226, 228 is then coupled to a multiplier (230, 232)for multiplication with a respective carrier. The resultant waveformsare then summed at summing node 234 and transmitted over thecommunication channel to the receiver. At the receiver, an analogpre-processing unit 242 receives the transmitted signal, which isprocessed and passed to a matched filter 244 The output of the matchedfilter 244 is then provided to an Analog/Digital (A/D) converter 246.Note that other modules may be implemented according to design andoperational criteria. The components and elements of FIGS. 2A and 2B areprovided for an understanding of the following discussion and are notintended to be a complete description of a communication system.

RAKE and Equalizer Combination

In another design, a RAKE receiver is operated in parallel with anequalizer. Such a design is detailed in “METHOD AND APPARATUS FORPROCESSING A MODULATED SIGNAL USING AN EQUALIZER AND A RAKE RECEIVER,”by John Smee et al., having application Ser. No. 09/624,319, filed Jul.24, 2000. A selection is made between the RAKE receiver and theequalizer to determine the best estimate of the received signal. Forexample, the selection may correspond to the lowest Mean Square Error(MSE) between a transmitted pilot signal and the estimate, or thehighest Signal to Interference and Noise Ratio (SINR) at each output, orsome other criteria. The performance measure or estimate provides ameans for comparing the RAKE and the equalizer. The selected receiverconfiguration is then used for processing the received data signal.

A receiver is termed “universal” if its performance is optimum over the“universe” of possible channel conditions and rates of channelvariation. The receiver with a RAKE and an equalizer is “universal” ifthe receiver configuration selected on the basis of the MSE estimate orC/I estimate is, in fact, the best configuration among the twoconfigurations. Thus accurate MSE estimates or C/I estimates arenecessary to make a receiver “universal.”

FIG. 3 is a block diagram of receive data processor 310 within a mobilestation 300 in accordance with an embodiment of the invention. In thisembodiment, receive data processor 310 includes two signal processingpaths that can be operated in parallel to provide improved performance,especially at higher data rates. The first signal processing pathincludes an equalizer 312 coupled to a post processor 314 and the secondsignal processing path includes a RAKE 316.

Within receive data processor 310, the streams of samples frompre-processors (not shown) are provided to each of equalizer 312 andRAKE 316. Each stream of samples is generated from a respective receivedsignal, wherein the received signal is routed from antennas 302 toreceiver 304. Equalizer 312 performs equalization on the receivedstreams of samples and provides symbol estimates to post processor 314.Depending on the processing performed at transmission, post processor314 may further process the symbol estimates to provide recoveredsymbols. In particular, if PN spreading and covering are performed atthe transmitter unit, post processor 314 may be configured to performdespreading with a complex PN sequence and decovering with one or morechannelization codes. Phase rotation (which is achieved via pilotdemodulation for a rake receiver) is implicitly achieved by equalizer312 after the filter coefficients have been adopted.

RAKE 316 may be configured to process one or more multipaths of eachreceived signal to provide recovered symbols for that received signal.For each stream of samples, RAKE 316 may be configured to perform PNdespreading, decovering, and coherent demodulation for a number ofmultipaths. RAKE 316 then combines demodulated symbols for allmultipaths of a received signal to generate recovered symbols for thatreceived signal. RAKE 316 may further combine the recovered symbols forall received signals to provide the overall recovered symbols that areprovided from the rake receiver.

The recovered symbols from post processor 314 and RAKE 316 may beprovided to a switch (SW) 320 that selects the recovered symbols fromeither post processor 314 or RAKE 316 to provide to a de-interleaver322. The selected recovered symbols are then reordered by de-interleaver322 and subsequently decoded by a decoder 324. A controller 318 couplesto, and manages the operation of equalizer 312, post processor 314, rakereceiver 316, and switch 320.

In accordance with the invention, equalizer 312 may be used to provideequalization of the received signals to reduce the amount of ISI in thereceived signals. Each received signal is distorted by thecharacteristics of the transmitter unit, the transmission channel, andthe receiver unit. Equalizer 312 may be operated to equalize the overallresponse for each received signal, thus reducing the amount of ISI. Thelower ISI improves S/N and may support higher data rates.

Continuing with FIG. 3, receive data processor 310 includes two signalprocessing paths that can be operated to process the received signals.The first signal processing path includes equalizer 310 and postprocessor 314, and the second signal processing path includes RAKE 316.In an embodiment, the two signal processing paths can be operated inparallel (e.g., during the adaptation period) and a signal qualityestimate can be computed for each of the signal processing paths. Thesignal processing path that provides the better signal quality can thenbe selected to process the received signals.

For a conventional RAKE, the received signal quality can be estimated bycomputing the signal-to-noise (SIN) ratio. For CDMA systems thattransmit TDM pilot reference, the S/N can be computed during the pilotreference period when the received signal is known. A signal qualityestimate can be generated for each assigned finger processor. Theestimates for all assigned finger processors can then be weighted andcombined to generate an overall S/N, which can be computed as:

$\begin{matrix}{{S/N_{RAKE}} = \frac{\left( {\sum\limits_{i = 1}^{K}\;{\beta_{i} \cdot \sqrt{E\; s_{i}}}} \right)^{2}}{\sum\limits_{i = 1}^{K}\;{{\beta_{i}^{2} \cdot N}\; t_{i}}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where β is the weighting factors used by the rake receiver to combinethe demodulated symbols from the assigned finger processors to providethe recovered symbols that are improved estimates of the transmitteddata, Es is the energy-per-symbol for the desired signal (e.g., thepilot) and Nt is the total noise on the received signal being processedby the finger processor. Nt typically includes thermal noise,interference from other transmitting base stations, interference fromother multipaths from the same base station, and other components. Theenergy-per-symbol can be computed as:

$\begin{matrix}{{{E\; s} = {\frac{1}{N_{SYM}}{\sum\limits_{i = 1}^{N_{SYM}}\;\left( {{P_{I}^{2}(i)} + {P_{Q}^{2}(i)}} \right)}}},} & {{Eq}.\mspace{14mu}(2)}\end{matrix}$

where P_(I) and P_(Q) are the in phase and quadrature filtered pilotsymbols and N_(SYM) is the number of symbols over which the energy isaccumulated to provide the Es value. The filtered pilot symbols can begenerated by accumulating the despread samples over the length of thechannelization code used to cover the pilot reference. The total noisecan be estimated as the energy of the variations in the energy of thedesired signal, which can be computed as:

$\begin{matrix}{{N\; t} = {\frac{1}{N_{SYM} - 1}{\left\{ {{\sum\limits_{i = 1}^{N_{SYM}}\;\left( {{P_{I}^{2}(i)} + {P_{Q}^{2}(i)}} \right)} - {\frac{1}{N_{SYM}}\left( {\sum\limits_{i = 1}^{N_{SYM}}{P_{I}(i)}} \right)^{2}} - {\frac{1}{N_{SYM}}\left( {\sum\limits_{i = 1}^{N_{SYM}}{P_{Q}(i)}} \right)^{2}}} \right\}.}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$

The measurement of the received signal quality is described in furtherdetail in U.S. Pat. No. 5,903,554, entitled “METHOD AND APPARATUS FORMEASURING LINK QUALITY IN A SPREAD SPRECTRUM COMMUNICATION SYSTEM,” andU.S. Pat. No. 5,799,005, entitled “SYSTEM AND METHOD FOR DETERMININGRECEIVED PILOT POWER AND PATH LOSS IN A CDMA COMMUNICATION SYSTEM,” bothassigned to the assignee of the present invention.

For the signal processing path that includes equalizer 312, the signalquality may be estimated using various criteria, including a mean squareaverage (MSE). Again, for CDMA systems that transmit TDM pilotreference, the MSE can be estimated during the pilot reference period,and can be computed as:

$\begin{matrix}{{{MSE} = {\frac{1}{N_{SAM}}{\sum\limits_{n = 1}^{N_{SAM}}{{{y(n)} - {\hat{y}(n)}}}^{2}}}},} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$

where N_(SAM) is the number of samples over which the error isaccumulated to provide the MSE. Typically, the mean square error isaveraged over a number of samples, and over one or more pilotreferences, to obtain a desired level of confidence in the measurement.The mean square error can then be translated to an equivalentsignal-to-noise ratio, which can be expressed as:

$\begin{matrix}\begin{matrix}{{S/N_{EQ}} = {\frac{1}{MSE} - {1\mspace{20mu}{linear}}}} \\{= {10\;{\log\left( {\frac{1}{MSE} - 1} \right)}\mspace{14mu}{dB}}}\end{matrix} & {{Eq}.\mspace{14mu}(5)}\end{matrix}$

The S/N_(EQ) for the signal processing path with equalizer 312 can becompared with the S/N_(RAKE) for the signal processing path with RAKE316. The signal processing path that provides the better S/N can then beselected to process the received signals.

Alternatively, the MSE can be computed for the signal processing pathwith RAKE 316 and compared against the MSE computed for the signalprocessing path with equalizer 312. The signal processing path with thebetter MSE may then be selected.

For a HRPD system, the S/N is estimated at a remote terminal and used todetermine a maximum data rate that can be received by the remoteterminal for the operating conditions. The maximum data rate is thentransmitted back to the base station for which the S/N is estimated.Thereafter, that base station transmits to the remote terminal at a datarate up to identified maximum data rate.

With the present invention, the data rate for a data transmission can beestimated using various methods. In one method, the S/N can be estimatedfor the RAKE or for the equalizer based on the computed MSE. The bestS/N from all signal processing paths can then be used to determine amaximum data rate that can be supported. Alternatively, the MSE can beused to directly determine the maximum data rate. The best S/N, MSE, ormaximum data rate can be sent to the base station.

Under certain operating conditions, the signal processing path with theequalizer can provide better performance than the one with the rakereceiver. For example, the signal processing path with the equalizertypically performs better when the S/N is high and for channels withISI. The RAKE can be used to process multipaths, which also cause ISI.In fact, the RAKE can be viewed as a filter with L taps (where Lcorresponds to the number of finger processors), with each tapcorresponding to a time delay that can be adjusted. However, the RAKE isnot as effective at reducing ISI due to frequency distortion in thereceived signals.

The equalizer may more effectively reduce ISI due to frequencydistortion. This is achieved by providing a response that isapproximately the inverse of the frequency distortion while attemptingto minimize the overall noise, which includes the ISI. The equalizerthus “inverts” the channel and also attempts to smooth out the effect ofmultipath. In fact, each filter, when the coefficients are initializedto {0, . . . , 0, 1, 0, . . ., 0}, is equivalent to one fingerprocessor. Subsequently, as the zero-valued coefficients are adapted,the filter frequency response is altered to equalize the channeldistortion. Thus, the equalizer may be used to effectively deal withboth multipath-induced ISI and channel-induced ISI.

For simplicity, many of the aspects and embodiments of the inventionhave been described for a spread spectrum communication system. However,many of the principles of the invention described herein can be appliedto non- spread spectrum communication systems, and communication systemscapable of selectively performing direct sequence spreading, such as theHRPD system.

RAKE and Hybrid Equalizer Configuration

According to one embodiment, the equalizer 312 may be a hybridequalizer, wherein equalizer 312 is turned on when operating conditions,including but not limited to channel conditions, encourage the user ofan equalizer. In other words, when the equalizer 312 is expected toperform as well as or better than the RAKE 316, then the equalizer 312is turned on. Else, the equalizer is not operated. In this way, thesystem experiences power savings during those times when the equalizeris expected to perform worse than the RAKE 316. Such an equalizer isreferred to as a “hybrid” equalizer, as the equalizer is responsive tooperating conditions.

A hybrid equalizer and RAKE receiver architecture operates by comparingan operating condition metric, such as the potential demodulation SINRoutputs of a RAKE and equalizer, and then selecting the mode thatachieves the best performance. Modes may include, but are not limitedto, RAKE only mode, and RAKE and equalizer mode. One embodiment includesa test mode to periodically run the equalizer and select between theRAKE only and RAKE and equalizer modes. The hybrid equalizer willtypically have better performance for higher geometry and slow fadingconditions. In such conditions, the equalizer offers performance gainsas compared to a conventional RAKE only design. In the simplestimplementation, however, the cost of running both methods may beprohibitive, incurring increased power dissipation even for thoseconditions for which the equalizer offers no gain over the RAKE.

Ideally the equalizer only operates when gains in performance may berealized. The hybrid equalizer provides power reduction by employing adecision algorithm based on short temporal operating conditions, such ascorrelation statistics and/or receiver SINR. The hybrid equalizer isoperated only when the channel conditions are likely to yieldperformance gains.

As the equalizer relies on slow fading channel conditions, oneembodiment estimates fading dynamics by estimating the inter-pilot burstcorrelation statistics. An equalizer typically yields gains for highergeometry (i.e., SINR), wherein another embodiment estimates SINR fromthe pilot burst. The two metrics may both be used in a decisionalgorithm. If the correlation metric is above a given threshold and theSINR is also above another threshold, then the equalizer is enabledotherwise the equalizer is disabled. This reduces the power dissipationby avoiding the use of the equalizer when no benefit is achieved.

FIG. 5 is a state diagram illustrating operation 500 of a receiveraccording to one embodiment implementing a RAKE and hybrid equalizer.Two modes are implemented: RAKE only mode 502; and RAKE and hybridequalizer mode 504. Operation starts in the RAKE only mode. While inmode 502, operation is maintained in mode 502 until there is a change inoperating conditions sufficient to indicate performance increases wouldbe achieved with addition of the equalizer. To determine if operatingconditions have changed sufficiently, such as a slowly varying channelcondition, a channel quality metric is evaluated. In the presentembodiment, the channel quality metric is the Signal to Interference andNoise Ratio (SINR) of the RAKE output (SINR_(RAKE)) is measured andcompared to a threshold value (T_(EQU)) for triggering the equalizer.Similarly, a correction metric (C_(RAKE)) is determined for the RAKE andcompared to a corresponding correction metric (C_(EQU)) for theequalizer. When SINR_(RAKE) is greater than T_(EQU), and C_(RAKE) isgreater than C_(EQU)then operation transitions to a RAKE and equalizermode 504. In this way, when operating conditions encourage use of theequalizer, mode 504 is entered and the equalizer operation begins.

While in mode 504 the system continues to monitor the SINR of the RAKEoutput and the equalizer output (SINR_(EQU)). When SINR_(RAKE) isgreater than SINR_(EQU), operation transitions to mode 502. The use ofan equalizer is typically encouraged as the SINR increases, as SINR (asa function of current geometry of the system) indicates the channelcondition. For low SINR, the equalizer does not perform as well, andtherefore SINR acts as a good trigger for turning the equalizer on andoff. The trigger for entering the mode 504, i.e., enabling theequalizer, is effectively a two-part consideration. The first evaluationdetermines if the channel condition, e.g., SINR, is consistent withthose conditions for which equalizer operation improves performance. Thesecond evaluation determines the speed of the channel, or in otherwords, how quickly a mobile station is moving in the cellular network.According to one embodiment, the second evaluation determines thecross-correlation between pilot bursts. Cross-correlation measures thedegree to which two series are correlated. In this case, as thecorrelation of the signals increases, the delay between the two signalsdecreases. Similarly, the correlation decreases as the delay increases.Therefore, as the distance between the mobile station and the receiverincreases, or changes, there is a decrease in the correlation of signalsreceived. For a low cross-correlation, the equalizer is enabled in mode504, else the RAKE only mode 502 is maintained. The cross-correlationmay be measured on the pilot signal, or pilot burst, as this is a knownsignal providing confidence in the result.

As an example, consider the cross correlation metric as follows. Given apilot symbol, Pμ, the correlation between successive pilot symbols maybe estimated as:

$\begin{matrix}{C_{RAKE} = {\frac{\sum\limits_{k = 1}^{N_{SUM}}\;{P_{\mu}P_{k + 1}^{*}}}{\sum\limits_{\mu = 1}^{N_{SUM}}\;{P_{\mu}P_{\mu}^{*}}}}} & {{Eq}.\mspace{14mu}(6)}\end{matrix}$by averaging over N_(SUM) pilot symbols. The correlation metric rangesfrom 0 to 1. Correlation of 1 implies a strong correlation and is likelyto yield good equalizer performance, as the channel is not changingbetween successive pilot symbols.

Alternate embodiments may define other parameters and metrics, whichtrigger an equalizer. Metrics may be selected which provide an expectedequalizer performance. The metrics may be selected specific to thesystem design and performance goals.

FIG. 6 is a state diagram of operation 520 for an alternate embodimenthaving three modes of operation. In a RAKE only mode 522 the RAKE isused while the equalizer is not operating. Periodically, measured by asample period, which may be prespecified or adaptive, operation enters atest mode 524.

During the test mode 524, equalizer operation is enabled. The test mode524 enables the equalizer to determine if the performance of theequalizer adds to the performance of the receiver. The results of theRAKE and equalizer are compared to evaluate performance of theequalizer. When SINR_(RAKE) is less than SINR_(EQU), operationtransitions to RAKE and equalizer mode 526, wherein both the RAKE andequalizer are enabled. In this situation, the equalizer shows capacityfor improving performance. If the results in test mode 524 indicate thatSINR_(RAKE) is greater than SINR_(EQU), operation transitions back tomode 522 for RAKE only. In this situation the equalizer does not improveperformance, and is not expected to provide an overall improvement inperformance under the current conditions. Note, a margin value (δ) maybe illustrated, wherein SINR_(RAKE) or SINR_(EQU) is biased according tosystem design and/or performance. The sample period may be designed as afunction of the time required to operate the equalizer, wherein thesample period is sufficient to allow data to traverse the filterelements of the equalizer.

While in mode 526, the system monitors the channel quality metric. WhenSINR_(RAKE) is greater than SNR_(EQU), operation transitions to RAKEonly mode 522. Note that transitions implement a margin value (δ) so asto avoid toggling between modes. In this way, transitions from mode 524to mode 526 occur when the SINR of the estimate generated by theequalizer exceeds the SINR generated by the RAKE by more than a margin.Similarly, transitions from mode 524 to mode 522 occur when the SINR ofthe estimate generated by the RAKE exceeds the SINR generated by theequalizer by more than a margin. Additionally, transitions from mode 526to mode 522 occur when the SINR of the estimate generated by the RAKEexceeds the SINR generated by the equalizer by more than a margin.

High Rate Packet Data Communication Systems

Throughout the following discussion a specific high data rate system isdescribed for clarity. Alternate systems may be implemented that providetransmission of information at high data rates. For CDMA communicationssystems designed to transmit at higher data rates, such as a High RatePacket Data (HRPD) or High Data Rate (HDR) communications system, avariable data rate request scheme may be used to communicate at themaximum data rate that the C/I may support. The HDR communicationssystem is typically designed to conform to one or more standards such asthe “cdma2000 High Rate Packet Data Air Interface Specification,” 3GPP2C.S0024, Version 2, Oct. 27, 2000, promulgated by the consortium “3^(rd)Generation Partnership Project.”

Generally, in an HRPD system, an Access Network (AN) is defined as thenetwork equipment providing data connectivity between a cellular networkand a packet switched data network (typically the Internet) and the ATs.An AN in an HRPD system is equivalent to a base station in a cellularcommunication system. An Access Terminal (AT) is defined as a deviceproviding data connectivity to a user. An AT in an HRPD systemcorresponds to a mobile station in a cellular communication system. AnAT may be connected to a computing device such as a laptop personalcomputer or it may be a self-contained data device such as a PersonalDigital Assistant (PDA). Note that the terms mobile station, remoteterminal, and access terminal are used interchangeably.

A receiver in an exemplary HDR communications system employing avariable rate data request scheme is shown in FIG. 4. The receiver 400is a subscriber station in communication with a land-based data networkby transmitting data on a reverse link to a base station (not shown).The base station receives the data and routes the data through a basestation controller (BSC)(also not shown) to the land-based network.Conversely, communications to the subscriber station 400 may be routedfrom the land-based network to the base station via the BSC andtransmitted from the base station to the subscriber unit on the forwardlink. The forward link refers to the transmission from the base stationto the subscriber station and the reverse link refers to thetransmission from the subscriber station to the base station.

In the exemplary HDR communications system, the forward link datatransmission from the base station to the subscriber station 400 shouldoccur at or near the maximum data rate which may be supported by theforward link. Initially, the subscriber station 400 establishescommunication with the base station using a predetermined accessprocedure. In this connected state, the subscriber station 400 mayreceive data and control messages from the base station, and is able totransmit data and control messages to the base station. The subscriberstation 400 then estimates the C/I of the forward link transmission fromthe base station 400. The C/I of the forward link transmission may beobtained by measuring the pilot signal from the base station. Based onthe C/I estimation, the subscriber station 400 transmits to the basestation a data rate request message as a Data Rate Control (DRC) messageon an assigned DRC channel. The DRC message may contain the requesteddata rate or, alternatively, an indication of the quality of the forwardlink channel, e.g., the C/I measurement itself, the bit-error-rate, orthe packet-error-rate. The base station uses the DRC message from thesubscriber station 400 to efficiently transmit the forward link data atthe highest possible rate.

The BSC (not shown) may interface with a packet network interface, aPSTN, and/or other base stations, and serves to coordinate thecommunication between subscriber stations and other users.

The forward link pilot channel provides a pilot signal, which may beused by the subscriber station 400 for initial acquisition, phaserecovery, and timing recovery. In addition, the pilot signal may also beused by subscriber station 400 to perform the C/I measurement. In thedescribed exemplary embodiment, each time slot on the forward link is2048 chips long with two pilot bursts occurring at the end of the firstand third quarters of the time slot. Each pilot burst is 96 chips induration. Each slot has two parts, wherein each half slot includes apilot burst.

The forward link transmission is received by an antenna at thesubscriber station 400. The received signal is routed from the antennato a receiver within analog preprocessing unit 402, matched filter 404,and Analog to Digital (A/D) converter 406. The receiver filters andamplifies the signal, downconverts the signal to baseband, quadraturedemodulates the baseband signal, and digitizes the baseband signal. Thedigitized baseband signal is coupled to a demodulator. The demodulatorincludes carrier and timing recovery circuits and further includes theequalizer 410. The equalizer 410 compensates for ISI and generatessymbol estimates from the digitized baseband signal. The symbolestimates are coupled to a controller 416 via communication bus 420. Thecontroller then generates the DRC message. The output of the equalizer410 is also provided to decoder 412. The decoder 412, the equalizer 410,and the controller 416 are each coupled to communication bus 420.

In addition to generating the DRC message, the controller 416 may beused to support data and message transmissions on the reverse link. Thecontroller 416 may be implemented in a microcontroller, amicroprocessor, a digital signal processing (DSP) chip, an ASICprogrammed to perform the function described herein, or any otherimplementation known in the art. A timing unit 414 is also coupled tothe communication bus 420. The exemplary embodiment includes a samplememory storage unit 408 coupled to the equalizer 410 and the controller416 via the communication bus 420.

A RAKE 418 is also coupled to the communication bus 420 and receivesinputs for processing via a structure such as illustrated in FIG. 1. Anequalizer controller 422 receives the estimates from the RAKE 418, andfrom the hybrid equalizer 410 when operating. The equalizer controller422 then determines when the equalizer is to be used and initiatesoperation. Similarly, equalizer controller 422 determines when theequalizer is not to be used and initiates termination of operation.Various monitoring units may be implemented to check operating metrics,such as channel quality and/or channel velocity. The equalizercontroller 422 uses such information to make equalizer decisions.

Performance Measurement

As described hereinabove, the equalizer configuration may be selectedbased on a measurement of the SINR, C/I or other performance criteria.Other performance criteria may include, for example, the Mean SquareError of the equalizer configuration measured on the pilot samples. Forexample, if the equalizer outputs on pilot samples are given by{{circumflex over (γ)}_(n):n=1, . . . , K} and the desired pilot symbolsare denoted by {γ_(n):n=1, . . . , K}, the Mean Square Error (MSE) forthis configuration is given by:

$\begin{matrix}{{MSE} = {\frac{1}{K}{\sum\limits_{n = 1}^{K}\;{{{{\hat{y}}_{n} - y_{n}}}^{2}.}}}} & {{Eq}.\mspace{14mu}(7)}\end{matrix}$One definition of the SINR or C/I estimate is the following:

$\begin{matrix}{{SINR} = {\frac{1}{MSE} - 1.}} & {{Eq}.\mspace{14mu}(8)}\end{matrix}$Other definitions or performance measures are also possible.

The models, methods, and apparatus presented hereinabove serve asexamples of various embodiments supporting different systems, channelconditions, and receiver designs. The application of parallel equalizersas described hereinabove may be implemented in any of a variety ofreceivers adapted for operation in a variety of communication systems,including but not limited to high data rate systems.

Those skilled in the art will appreciate that the various illustrativelogical blocks, modules, circuits, and algorithms described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, andalgorithms have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The methods or algorithms described in connection with the embodimentsdisclosed herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processormay read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of receiving data in a wireless communication system,comprising: comparing a first metric associated with a RAKE processingelement to a second metric associated with an equalizer, wherein saidfirst and second metrics are respective wireless communication channelmetrics; and based on said comparing, determining whether to transitionfrom one of first and second modes of data reception to the other ofsaid first and second modes of data reception; wherein said first modeof data reception is defined by a first combination of respectiveoperational states of the RAKE processing element and the equalizer;wherein said second mode of data reception is defined by a secondcombination of respective operational states of the RAKE processingelement and the equalizer; wherein said first combination of operationalstates differs from said second combination of operational states; andwherein said wireless communication channel metrics are channel speedmetrics.
 2. The method of claim 1, wherein each of said channel speedmetrics includes signal correlation information.
 3. A method ofreceiving data in a wireless communication system, comprising: comparinga first metric associated with a RAKE processing element to a secondmetric associated with an equalizer; and based on said comparing,determining whether to transition from one of first and second modes ofdata reception to the other of said first and second modes of datareception; wherein said first mode of data reception is defined by afirst combination of respective operational states of the RAKEprocessing element and the equalizer; wherein said second mode of datareception is defined by a second combination of respective operationalstates of the RAKE processing element and the equalizer; wherein saidfirst combination of operational states differs from said secondcombination of operational states; and wherein said comparing includescomparing one of said first and second metrics to a biased version ofthe other of said first and second metrics.
 4. The method of claim 3,wherein said one metric is said first metric.
 5. The method of claim 3,wherein said one metric is said second metric.
 6. A method of receivingdata in a wireless communication system, comprising: comparing a firstmetric associated with a RAKE processing element to a second metricassociated with an equalizer; and based on said comparing, determiningwhether to transition from one of first and second modes of datareception to the other of said first and second modes of data reception;wherein said first mode of data reception is defined by a firstcombination of respective operational states of the RAKE processingelement and the equalizer; wherein said second mode of data reception isdefined by a second combination of respective operational states of theRAKE processing element and the equalizer; wherein said firstcombination of operational states differs from said second combinationof operational states and; wherein said one mode of data reception is aperiodically activated test mode in which the RAKE processing elementand the equalizer are enabled for operation concurrently.
 7. The methodof claim 6, wherein the RAKE processing element is enabled for operationand the equalizer is disabled from operation in said other mode of datareception.
 8. The method of claim 6, including periodicallytransitioning from said other mode of data reception to said one mode ofdata reception.
 9. The method of claim 8, wherein the RAKE processingelement is enabled for operation and the equalizer is disabled fromoperation in said other mode of data reception.
 10. A wirelesscommunication apparatus, comprising: an input for receiving data signalsvia a wireless communication link; a RAKE processing element coupled tosaid input; an equalizer coupled to said input and co-operable with saidRAKE processing element to define first and second modes of datareception in said wireless communication apparatus; and a controllercoupled to said RAKE processing element and said equalizer, saidcontroller making a determination of whether said wireless communicationapparatus is to transition from one of said first and second modes ofdata reception to the other of said first and second modes of datareception, said controller making said determination based on acomparison of a first metric associated with said RAKE processingelement to a second metric associated with said equalizer; wherein saidfirst mode of data reception is defined by a first combination ofrespective operational states of the RAKE processing element and theequalizer; wherein said second mode of data reception is defined by asecond combination of respective operational states of the RAKEprocessing element and the equalizer; and wherein said first combinationof operational states differs from said second combination ofoperational states.
 11. The apparatus of claim 10, wherein the RAKEprocessing element and the equalizer are enabled for operationconcurrently in said first mode of data reception.
 12. The apparatus ofclaim 11, wherein the RAKE processing element is enabled for operationand the equalizer is disabled from operation in said second mode of datareception.
 13. The apparatus of claim 10, wherein said one mode of datareception is a periodically activated test mode in which the RAKEprocessing element and the equalizer are enabled for operationconcurrently.
 14. The apparatus of claim 13, wherein the RAKE processingelement is enabled for operation and the equalizer is disabled fromoperation in said other mode of data reception.
 15. The apparatus ofclaim 13, including periodically transitioning from said other mode ofdata reception to said one mode of data reception.
 16. The apparatus ofclaim 15, wherein the RAKE processing element is enabled for operationand the equalizer is disabled from operation in said other mode of datareception.
 17. The apparatus of claim 10, wherein said first and secondmetrics are respective wireless communication channel metrics.
 18. Theapparatus of claim 17, wherein said wireless communication channelmetrics are channel quality metrics.
 19. The apparatus of claim 18,wherein each of said channel quality metrics includes signal-to-noiseratio information.
 20. The apparatus of claim 17, wherein said wirelesscommunication channel metrics are channel speed metrics.
 21. Theapparatus of claim 20, wherein each of said channel speed metricsincludes signal correlation information.
 22. The apparatus of claim 17,wherein each of said wireless communication channel metrics includessignal correlation information.
 23. The apparatus of claim 10, whereinsaid comparing includes comparing one of said first and second metricsto a biased version of the other of said first and second metrics. 24.The apparatus of claim 23, wherein said one metric is said first metric.25. The apparatus of claim 23, wherein said one metric is said secondmetric.